Yes, you can! There are few safe ways to wear a 5 point harness and still keep your baby swaddled for a happy transit from point to point. As much I hate the current car seat safety equipment (when will they invent a womb like bubble to keep the baby moving and cushioned from every angle at every possible moment???) I do like this adaptation of the ever popular swaddle for newborns.
If you can set up ahead of time it is the easiest. Take a thin cotton blanket--it can be stretchy or just thin flannel, but the key is to use something not bulky. (If you use the 'bat wings' swaddle or 'diaper swaddle' that we teach in our baby care classes and moms groups, this is just another way to use this approach.)
Lay the blanket on the bias (usually in a triangle) and then fold up the bottom tip so it doesn't interfere with the buckle. Then tuck each 'wing' under the straps and put the baby in. Once baby is properly fastened in (no pinch on the belting or you need to tighten more) then you can wrap the blanket around the arms.
As with swaddling, do each arm separately and adjust the position for the maturity of the baby (i.e. preemies get bent arms, very new babies get their hands up so we can see feeding cues, 'Houdini' older babies get arms straighter down). The finished product might look something like this:
There should not be a lot of thickness of material between the baby and the car seat itself. This could cause problems in an accident, so it is important to dress baby modestly and layer over the straps and swaddle for temp control.
Moms have reported at moms group that this technique is a lifesaver for being in traffic and not being able to attend to their babies. Until they invent a new system, it seems worth a try for babies who don't magically go to sleep as soon as they are in the car and moving. I hope this is a handy hint to send to your friends who have unhappy car travelers. I had one too, and I sure wish I would have figured this out a few years ago so we could have tried it with her!